The basic principle of identifying herbicides via inhibiting a defined target is known (for example U.S. Pat. No. 5,187,071, WO 98/33925, WO 00/77185). In general, there is a great demand for the detection of enzymes which might constitute novel targets for herbicides. Reasons for this are that herbicidal active ingredients which act on known targets demonstrate the development of resistance problems, and the constant endevor to identify novel herbicidal active ingredients which are distinguished by as broad as possible a range of action, environmental friendliness and toxicological compatibility and/or low application rates.
In practice, the detection of novel targets always entails great difficulties since the inhibition of an enzyme which is part of a metabolic pathway frequently has no further effects on the plant's growth. The reason may be that the plant switches over to alternative metabolic pathways whose existence is not known, or that the enzyme which is being inhibited is not limiting for the metabolic pathway. Furthermore, plant genomes are distinguished by a high degree of functional redundance. In the Arabidopsis thaliana genome, functionally equivalent enzymes are more frequently found in gene families than is the case with insects or mammals (Nature, 2000, 408(6814):796-815). This hypothesis is confirmed experimentally by the fact that large gene knock-out programs by means of the insertion of T-DNA or transposons into Arabidopsis have, as yet, yielded fewer manifested phenotypes than expected (Curr. Op. Plant Biol. 4, 2001, pp. 111-117).